This invention relates to low temperature cooling and, more particular, to pulse-tube cooling.
There are an increasing number of applications that require cooling to cryogenic temperatures, i.e., 110K. and lower. For example, high temperature superconductivity occurs at temperatures which are supported by liquid nitrogen, having a boiling point of 77K. A class of refrigerators known as regenerative cryocoolers provides a relatively simple refrigeration system for operating at these temperatures. Probably the best known example of a regenerative cryocooler is a Stirling refrigerator. A fluid operating medium is compressed, displaced, expanded, and returned by oscillatory devices at end volumes of the refrigerator adjacent a region of high heat capacity, the regenerator, wherein the compression, displacement, expansion, and return are in a phased relationship to transport heat from one end of the regenerator to the other. The regenerator includes a plurality of surfaces axially aligned between the end regions and having a spacing between surfaces which is much smaller than a thermal penetration depth (i.e., basically, the distance that heat diffuses during a cycle of the driving wave) to maintain the fluid temperature at the same temperature as the surface.
In an adaptation of the Stirling refrigerator, one of the oscillatory devices at one end of the engine is replaced with a "pulse" tube having a diameter which is a few thermal penetration depths. The pulse tube walls function to enable heat pumping to occur. The heat load, i.e., the "cold" heat exchanger, is located at the boundary between the regenerator and the pulse tube. "Hot" heat exchangers are located at the outer ends of the regenerator and the pulse tube to remove the transferred heat. A pulse tube refrigerator (PTR) is described in U.S. Pat. No. 3,237,421, issued Mar. 1966 to W. E. Gifford, incorporated herein by reference.
A refinement of the pulse tube refrigerator incorporates a large volume connected to the pulse tube by a flow impedance, e.g., an adjustable needle valve, for increasing the average fluid velocity throughout the engine, whereby the pulse tube pumps more heat to increase the total cooling power. An orifice pulse-tube refrigerator (OPTR) is described in R. Radebaugh, "Pulse Tube Refrigeration-A New Type of Cryocooler," 26 Jpn. J. Appl. Phys., Suppl. 26-3, page 2076 (1987), incorporated herein by reference. The OPTR described by Radebaugh still requires a mechanical compressor with its attendant sealing and mechanical problems and operates at a low frequency, around 10 Hz, and a high-amplitude pressure oscillation, on the order of 2-3 atm. However, operation of an OPTR has been reported to keep temperatures in the 60K. range.
It would clearly be preferable to provide an OPTR without having to rely on compressors or other devices with moving parts to generate the oscillatory pressure which drives the device. There is a class of engines which converts heat energy into acoustic energy with no moving parts. These thermoacoustic engines are described in U.S. Pat. Nos. 4,398,398, issued Aug. 16, 1983, to Wheatley et al., 4,489,553, issued Dec. 25, 1984, to Wheatley et al., and 4,722,201, issued Feb. 2, 1988, to Hofler et al. A review of these engines is further presented in an article by J. C. Wheatley et al., "The Natural Heat Engine," 14 Los Alamos Science No. 14, pp. 1-33, (1986) and in G. W. Swift, "Thermoacoustic Engines," 84 J. Acoust. Soc. Am. No. 4, pp. 1145-1180 (Oct. 1988). All of these references are incorporated herein by reference. The article by Wheatley et al., describes a "beer cooler" having both a thermoacoustic prime mover and a refrigerator. The acoustic output of the prime mover generates an acoustic standing wave effective for the refrigerator to generate some cooling. While it would appear desirable to incorporate a thermoacoustic prime mover for activating a pulse tube refrigerator, until the present invention the operating characteristics of a thermoacoustic prime mover were taught to be substantially different from those required by a pulse tube refrigerator: a thermoacoustic driver (TAD) operates at a high frequency, i.e., 500-600 Hz, and with low-amplitude pressure oscillations, i.e., 0.1-0.2 atm. versus an operating frequency of about 10 Hz and pressure amplitude on the order of 2 atm. for an OPTR.
These problems are overcome in the present invention in which a TAD and an OPTR are combined to form a cryocooler having no moving parts.
Accordingly, it is an object of the present invention to provide a pulse tube cryocooler with no moving parts.
Another object of the present invention is to provide a pulse tube cryocooler with operating characteristics which are compatible with a thermoacoustic prime mover.
One other object of the present invention is to provide a thermoacoustic prime mover which generates an acoustic wave at a frequency and pressure amplitude effective for generating low output temperatures in a pulse tube refrigerator.
Additional objects, advantages and novel features of the invention will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.